1. Field of the Invention
The present invention relates to a scrap stacking apparatus that can stack scraps generated upon executing processes on sheets, and detect its stacked state, and a sheet processing apparatus provided with such a scrap stacking apparatus.
2. Description of the Related Art
Conventionally, various sheet processing apparatuses provided with a processing portion that carries out a predetermined process on a sheet on which an image is formed by an image forming apparatus have been proposed. For the processing portion, for example, a perforating portion for carrying out a perforating process on a sheet and a shearing portion for carrying out a shearing process on a sheet bundle that has been saddle-stitched and folded in the middle to be formed into a book have been known. In the sheet processing apparatus having such a processing portion, since sheet scraps are generated by the perforating portion and the shearing portion, a sheet scrap stacking apparatus used for stacking the sheet scraps is installed. In such a sheet scrap stacking apparatus, a sheet scrap detection device composed of an oscillation portion and a receiving portion for detecting a fully stacked state of the sheet scraps is installed, as disclosed in Japanese Patent Laid-Open No. 2001-293691. In this sheet scrap detection device, an electromagnetic wave, emitted by the oscillation portion, is directed into a dust box through an incident port for the electromagnetic wave formed on the dust box to be used for housing sheet scraps. When the dust box is fully filled with the sheet scraps, the electromagnetic wave that has been directed thereto is blocked by the sheet scraps to prevent it from reaching an exit port and the receiving portion so that the fully filled state of the sheet scraps is detected.
In the above-mentioned conventional art, however, the incident port and the exit port that allow the electromagnetic wave to pass therethrough need to be formed on the dust box. In the case where holes are formed as the incident and exit ports, there is a possibility that sheet scraps might leak through these holes to be scattered externally. In contrast, in the case where electromagnetic wave transmitting members are attached to the incident and exit ports, although scattering of sheet scraps outside the dust box can be prevented, sheet scraps that have been charged tend to adhere to the incident and exit ports, with the result that the fully stacked state might be detected prior to an actual fully filled state.